Re: [PATCHv4 net 1/2] ipv4/icmp: fix rt dst dev null pointer dereference

["Jonathan Lemon" <jlemon@flugsvamp.com>]

On 22 Aug 2019, at 7:19, Hangbin Liu wrote:

> In __icmp_send() there is a possibility that the rt->dst.dev is NULL,
> e,g, with tunnel collect_md mode, which will cause kernel crash.
> Here is what the code path looks like, for GRE:
>
> - ip6gre_tunnel_xmit
>   - ip6gre_xmit_ipv4
>     - __gre6_xmit
>       - ip6_tnl_xmit
>         - if skb->len - t->tun_hlen - eth_hlen > mtu; return -EMSGSIZE
>     - icmp_send
>       - net = dev_net(rt->dst.dev); <-- here
>
> The reason is __metadata_dst_init() init dst->dev to NULL by default.
> We could not fix it in __metadata_dst_init() as there is no dev supplied.
> On the other hand, the reason we need rt->dst.dev is to get the net.
> So we can just try get it from skb->dev when rt->dst.dev is NULL.
>
> v4: Julian Anastasov remind skb->dev also could be NULL. We'd better
> still use dst.dev and do a check to avoid crash.
>
> v3: No changes.
>
> v2: fix the issue in __icmp_send() instead of updating shared dst dev
> in {ip_md, ip6}_tunnel_xmit.
>
> Fixes: c8b34e680a09 ("ip_tunnel: Add tnl_update_pmtu in ip_md_tunnel_xmit")
> Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>

Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>